The present invention relates to a piezoelectric acceleration pickup with a polymer transducer member.
Conventionally, a piezoelectric acceleration pickup has two main components. The first component is a piezoelectric transducer in the form of a flat member, wherein one face is fixed to the case containing the pickup, whilst the second component is a mass M, called the seismic mass, which is fixed to the other face of the transducer member. The overall arrangement is such that under the effect of an acceleration g, the mass M exerts a force Mg on the piezoelectric member which then supplies to a measuring circuit a charge quantity proportional to Mg, i.e. to g, M being known.
The prior art piezoelectric material was generally a polarized ferroelectric ceramic having a high piezoelectric coefficient d (expressed in coulomb/Newton). The response of these pickups is dependent on the temperature. The piezoelectric type material can be a ferroelectric monocrystal or even monocrystalline quartz when maximum importance is attached to the independence of the output voltage of the pickup with respect to the temperature. Pickups made by means of these materials and in particular the two latter types of materials are expensive. Moreover, certain pickups supply very low amplitude output signals, necessitating complex electronic matching and amplification circuits which contribute to increasing the price thereof.
Recently, piezoelectric polymers have become available, whose piezoelectric coefficients d are intermediate between those of quartz (1 to 5 10.sup.-12 C N.sup.-1) and those of ferroelectrics (100 to 500 10.sup.-12 C N.sup.-1). As a non-limitative example, reference is made to polyvinylidene fluoride (PVF.sub.2 : d=10 to 20 10.sup.-12 C.N.sup.-1), polyvinylidene fluoride/polytetrafluoroethylene copolymer (PVF.sub.2 -PTFE: d=5 to 10 10.sup.-12 C N.sup.-1), polyvinylchloride (PVC: d=1 to 5 10.sup.-12 C N.sup.-1) and polyvinylfluoride (PVF: d=2 to 7 10.sup.-12 C N.sup.-1). These polymers are made piezoelectric after shaping and melting by maintaining a temperature which is typically between 80.degree. and 120.degree. C. under applied electrical fields of 500 kV/cm to several MV/cm and polarization times between a few minutes and a few hours. The polymer is then brought to ambient temperature under an electrical field (e.g. in 30 minutes).
These polymer materials have the advantage of being very inexpensive and can be moulded into random shapes. However, for numerous applications and in particular for the general public, the need has arisen for a very inexpensive acceleration pickup. As non-limitative examples, reference is made to the detection of unbalance in rotary machines of the centrifuge type (e.g. the drum of washing machines) or the detection of shocks or impacts (e.g. shocks on closing doors), the detection of impacts between vehicles and obstacles, etc.